SMS Gateway

Whilst some balloon operators use SMS-based trackers to find out where their payloads land, these trackers don’t work at altitude so they don’t tell you anything about the path of the balloon or how high it got to. Also, if the payload lands somewhere outside mobile phone coverage, the payload is lost.

So almost everyone in the UK uses a different system based on UHF telemetry with listeners distributed around the country. This gives live telemetry throughout all, or almost all, of the flight. Overall it’s a much more reliable system than SMS, however part of the flight often has no telemetry. This is when the payload comes in to land – as it falls the radio horizon (the circle within which receivers need to be to receive the telemetry) gets smaller and smaller, so unless someone is very close then the signal will be lost until a chase car catches up. So, the actual landing spot is often unknown for a period of time.

It is also possible of course for the telemetry system to fail – e.g. the aerial breaks as the payload tumbles and the chute cords fly around the payload box on initial descent.

To solve both these problems, my next flight will contain a GSM module so that the flight computer can send out telemetry via text messages. Here’s the module, which is basically equivalent to a simple mobile phone sans display, keyboard and battery:

This connects to the flight computer via a TTL-RS232 level adapter. The flight software decides when to send a text, and at the moment the logic is “Every 30 seconds if riding or falling by at least 50 metres per minute, and every 5 minutes if not. Only send if below 2km altitude”. GSM only works below about 2km normally, so there’s no point it trying to send lots of texts above that altitude.

The texts simply contain the exact same telemetry string as the radio system uses. There’s a very good reason for this which we will cover below.

The texts are sent to a mobile phone which is connected to a PC back at base. The PC runs a program which connects to the phone, asking it every few seconds for any text messages that it has received:

If it has, then those messages are checked to see if they “look like” telemetry messages and, if so, are uploaded to the “habitat” system which drives the live tracking map. Once a message has been successfully uploaded it is deleted from the phone. So, assuming the payload is within GSM coverage, each message it sends will be relayed onto the map within 10-30 seconds. So everyone following the flight will quickly know where the payload has landed even if the chase car is not nearby, and the chase car will then have an exact location to aim for.

Here’s a complete track of the tracker destined for my CLOUD4 payload, showing my route to my dentist and back!